1. Field of the Invention
The present invention relates to a power supply apparatus, and especially relates to a power supply apparatus with an over power protection function.
2. Description of the Related Art
A power supply apparatus supplies power to an electronic apparatus to drive the electronic apparatus. The electronic apparatus informs the power supply apparatus of a required voltage (for examples, 5 volts or 20 volts) if the power supply apparatus supplies power through a universal serial bus interface.
The power supply apparatus comprises a transformer and a pulse width modulation controller. The transformer comprises a primary side winding, an auxiliary winding and a secondary side winding. When the electronic apparatus informs the power supply apparatus of the required voltage, the pulse width modulation controller changes a voltage of the primary side winding, and then the secondary side winding induces the voltage of the primary side winding to generate a voltage to send to the electronic apparatus. At the same time, the auxiliary winding induces the voltage of the secondary side winding to generate a voltage to drive the pulse width modulation controller.
Generally speaking, an output current of the power supply apparatus is limited to about 2 amperes when the power supply apparatus supplies power through the universal serial bus interface. Therefore, an output power of the power supply apparatus is limited. An output voltage of the power supply apparatus is increased to increase the output power of the power supply apparatus in a circumstance that the output current of the power supply apparatus is limited. For example, the output voltage of the power supply apparatus is increased from 5 volts to 20 volts.
Moreover, an over power protection function of the power supply apparatus is constant. Namely, the over power protection function of the power supply apparatus is designed for the output voltage 5 volts or 20 volts. The over power protection function of the power supply apparatus cannot operate smoothly when the output voltage of the power supply apparatus is 5 volts but the over power protection function of the power supply apparatus is designed for the output voltage 20 volts.
FIG. 3 shows a waveform diagram of the related art power supply apparatus that the over power protection function is designed for the output voltage 20 volts. FIG. 4 shows a waveform diagram of the related art power supply apparatus that the over power protection function is designed for the output voltage 5 volts. A trigger level of the over power protection function of the output voltage 5 volts is the same with a trigger level of the over power protection function of the output voltage 20 volts, so the output current of the output voltage 5 volts has to be greater than the output current of the output voltage 20 volts to achieve the trigger level of the over power protection function (the voltage times the current is equal to the power). However, if the output current is greater, power components in the power supply apparatus will work in a condition greater than a rated voltage or a rated current, so that the power components are damaged easily.